In process manufacturing, printing and allied arts, construction, shipbuilding, and many other facets of modern industry, there exists processes that require the drying or curing of liquid substances, such as inks, dyes and paints. In many of these applications there is an economic incentive to reduce the amount of time associated with such curing or drying stages, because such a reduction results in increased productivity.
Attempts have been made to reduce drying or curing time of liquid substances. Many attempts have been aimed at increasing the intrinsic volatility of the dryable liquids, often resulting in the evolution of noxious vapor, decreased flashpoint, increased chance of explosion and other undesirable properties of the hardened material.
In other instances, alternative chemistries to initiate curing have been developed including electron beam curing, ultraviolet curing, and two-photon curing. While many successful examples of such instances exist, there remain a large number of applications for which such processes are impractical. In other instances, focus has been placed on two-part chemistries. While two-part systems affect excellent curing properties in many systems and environments, they are impractical in other instances.
In many instances, focus has been paid to improving the curing apparatus itself. Many advances in printing press design, for example, have focused on increasing airflow and hence efficiency within drying tunnels, introduction of efficient ultraviolet (UV) emitters for UV-initiated inks, and the development of enhanced infrared (IR) drying mechanisms. While these approaches have often resulted in significantly improved printing press performance, they add cost and do not always result in sufficiently decreased cure time to fully meet all the needs of the printer.
Even with existing material and apparatus systems that nominally meet the needs of users, there are circumstances in which conditions are not conducive to drying and curing. In many cases, these adverse conditions relate to unusual environmental conditions that are too warm, too cool, too humid, or too dry. In other cases, variations in contacting surface properties such as changes in pH, porosity, or smoothness, for example, interfere with curing mechanisms. Altering these adverse surface properties to properties that are more conducive to curing is often not practical or desirable.
Within the field of printing, in particular, it is especially important to control curing or drying rates in order to maximize press speed and reduce waiting times between printing and post-processing steps. Many different technologies are used today for printing the goods, documents, and forms encountered daily. While much effort has been made in recent years to develop technologies that do not involve drying, such as electro-photography and thermal transfer, traditional wet ink technologies remain important.
Offset lithography is used for printing many of the documents and forms used on a daily basis. It is especially useful for short run documents such as business cards, flyers, pamphlets, etc.
Offset lithographic printing presses operate on the principal of immiscibility of polar and non-polar fluids. Printing plates used for offset lithographic printing are prepared with areas corresponding to printed areas having a hydrophobic property and areas corresponding to non-printed areas being hydrophilic. The printing plate is mounted on a cylinder, and the cylinder is rotated past a water delivery mechanism that coats the plate with water. Water stays on those areas that are hydrophilic and is repelled from the hydrophobic areas. The plate is then rotated further to an ink delivery mechanism that applies a thin layer of ink. The ink used by offset lithographic presses is oil-based and hydrophobic. Accordingly, the ink sticks to the hydrophobic areas of the plate where there is no water and does not stick to the hydrophilic areas coated with water.
After the plate is coated with ink adhering selectively, it is further rotated to a nip with the main cylinder. Paper is transported through the nip between the blanket cylinder and the impression cylinder in proper registration with the ink image on the blanket cylinder. Ink from the blanket cylinder is transferred to the paper at the main cylinder nip. After having ink thus adhered to its surface, the paper is then transported through a region where the ink cures, often on a chain that grips the edges of the paper. After, a curing device facilitates curing of the ink to make it dry enough to stack without transferring to the back of the overlying sheet, such transfers being known as offsetting. There are several different forms of curing devices including forced air drying tunnels, infrared (IR) emitters, electron beam emitters, and ultraviolet (UV) emitters.
Many offset lithographic printing presses are sheet fed as discussed above. In sheet-fed presses and duplicators, individual sheets are carried from the feeder, through one or more printing stations, through a drying or curing apparatus, and stacked in an output bin. Other offset lithographic presses and duplicators are web fed, the printing media being supplied in continuous web and carried in such a form throughout the printing press. Output from web-fed presses and duplicators may either be wound into a roll for further processing or may be cut, scored, folded, and/or stacked.
Referring now to the lithographic ink delivery mechanism, ink is delivered to the plate cylinder though an ink train. The ink train is comprised of a fountain containing bulk ink and a series of rollers that apply shear force, spread the ink, and physically move the resultant ink film to a nip where it is transferred to the plate cylinder. Shear force is used to level the ink, meter the ink, and reduce its viscosity sufficiently for further processing, including transfer to the plate cylinder, transfer to the blanket cylinder, transfer to the paper, and leveling on the paper.
Offset lithographic inks are generally high viscosity pastes that are shear thinned in the ink train. Conventional offset lithographic inks are heat-cured. Heat curing involves a complex sequence of events that includes boiling off volatile hydrocarbons, penetration of liquated phase materials into porous printing substrates, and cross-linking and oxidation of resin ink components. A more complete explanation of ink curing is presented in the book entitled What the Printer Should Know About Ink, by Terry Scarlett and Nelson R Eldred, published by the Graphics Arts Technical Foundation, hereby incorporated by reference. Curing often proceeds from the surface downward. Conventional heat-cured inks may be aided in curing by exposure to an IR emitter.
A common concern with the offset lithographic printing process and more specifically with the inks used in the process is the rate of cure of the ink. In many cases, ink in its pure form will not cure fast enough to meet process constraints. Often slip-sheets must be placed between stacked printed documents to prevent offsetting. Often printed sheets must be allowed to sit for a period of several hours or even days prior to further processing in order to prevent smudging of the printed image.
Many environmental and material variables affect the rate of cure including humidity, temperature, printing stock porosity and type, printing stock total alkalinity, and printing stock pH. Certain printing media, including felt-weave paper stock and non-porous plastics for example, have historically proven to be unsuitable for use in offset lithography because of poor drying. Some inks, for example those that contain green and blue pigments, are inherently slow drying. Other ink colors that frequently have relatively long drying times include violet, purple, opaque white, rhodamine, and process blue.
Drying agents have been added to inks in the ink fountain in order to speed curing. Drying agents are typically specialized so as to match particular printing stock and ink characteristics. Typical drying agents are hydrophobic, oil-based liquid that are added at a rate of fractional weight percentages, such as one-quarter to one-half ounce per pound of ink. In the prior art, drying agent concentration is kept low so as to avoid causing the ink to cure in the fountain or on the ink delivery train. Another disadvantage of the prior art ink driers is their tendency to reduce press runability, often causing stripping or splitting, the non-adherence of ink-to-ink train rollers, and other negative effects. Even after the addition of a drying agent to its recommended concentration, offset lithographic ink often does not dry fast enough to avoid the laborious process of adding slip sheets or the inconvenience of ageing the printed documents prior to further processing. Another common concern with lithographic printing is emulsification, or the formation of water phase around pigment particles during ink transfer from the ink train to the plate, then to the transfer cylinder, and on to the printing substrate. Emulsification results in print quality degradation and loss of tonal intensity.
Other widely used wet-ink printing technologies that have experienced issues with ink dry time include flexography, letterpress printing, and screen-printing. The use of UV-cured and electron beam-cured inks has been one approach that has resulted in harder drying and faster press running, but these types of inks are generally more expensive than more traditional heat-cured inks.
Flexographic and letterpress printing in particular have frequently been used to preprint label stock for use in heat-generating printings such as electrophotographic and thermal printers. Pre-printed media used in these applications has been especially sensitive to incomplete cure and is exhibited as ink transfer to internal printer components such as fuser rollers (in the case of electrophotographic printers) or printheads (in the case of thermal printers). Such unwanted ink transfer may frequently result in catastrophic failure of the affected components.
Another process that has frequently encountered drying or curing rates as an obstacle is painting. Often, paint drying rates are enhanced by heating the painted object and region. For some applications this is impractical owing to the size of the object or the environment. In other applications, such heating still results in drying times that are longer than desired. There are many industrial and domestic settings in which faster drying of paint is desirable.